/*
 * Calculate the checksum of data that is 16 byte aligned and a multiple of
 * 16 bytes.
 *
 * The first step is to reduce it to 1024 bits. We do this in 8 parallel
 * chunks in order to mask the latency of the vpmsum instructions. If we
 * have more than 32 kB of data to checksum we repeat this step multiple
 * times, passing in the previous 1024 bits.
 *
 * The next step is to reduce the 1024 bits to 64 bits. This step adds
 * 32 bits of 0s to the end - this matches what a CRC does. We just
 * calculate constants that land the data in this 32 bits.
 *
 * We then use fixed point Barrett reduction to compute a mod n over GF(2)
 * for n = CRC using POWER8 instructions. We use x = 32.
 *
 * http://en.wikipedia.org/wiki/Barrett_reduction
 *
 * This code uses gcc vector builtins instead using assembly directly.
 *
 * Copyright (C) 2017 Rogerio Alves <rogealve@br.ibm.com>, IBM
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of either:
 *
 *  a) the GNU General Public License as published by the Free Software
 *     Foundation; either version 2 of the License, or (at your option)
 *     any later version, or
 *  b) the Apache License, Version 2.0
 */
#include <stddef.h>
#include <altivec.h>


#define VMX_ALIGN	16
#define VMX_ALIGN_MASK	(VMX_ALIGN-1)

#ifdef REFLECT
static unsigned int crc32_align(unsigned int crc, const unsigned char *p,
			       unsigned long len)
{
	while (len--)
		crc = crc_table[(crc ^ *p++) & 0xff] ^ (crc >> 8);
	return crc;
}
#else
static unsigned int crc32_align(unsigned int crc, const unsigned char *p,
				unsigned long len)
{
	while (len--)
		crc = crc_table[((crc >> 24) ^ *p++) & 0xff] ^ (crc << 8);
	return crc;
}
#endif

static unsigned int __attribute__ ((aligned (32)))
__crc32_vpmsum(unsigned int crc, const void* p, unsigned long len);


unsigned CRC32_FUNCTION(unsigned crc, const void *buffer, size_t len)
{
	unsigned int prealign;
	unsigned int tail;

	const unsigned char *p = buffer;

#ifdef CRC_XOR
	crc ^= 0xffffffff;
#endif

	if (len < VMX_ALIGN + VMX_ALIGN_MASK) {
		crc = crc32_align(crc, p, len);
		goto out;
	}

	if ((unsigned long)p & VMX_ALIGN_MASK) {
		prealign = VMX_ALIGN - ((unsigned long)p & VMX_ALIGN_MASK);
		crc = crc32_align(crc, p, prealign);
		len -= prealign;
		p += prealign;
	}

	crc = __crc32_vpmsum(crc, p, len & ~VMX_ALIGN_MASK);

	tail = len & VMX_ALIGN_MASK;
	if (tail) {
		p += len & ~VMX_ALIGN_MASK;
		crc = crc32_align(crc, p, tail);
	}

out:
#ifdef CRC_XOR
	crc ^= 0xffffffff;
#endif

	return crc;
}

#if defined (__clang__)
#include "clang_workaround.h"
#else
#define __builtin_pack_vector(a, b)  __builtin_pack_vector_int128 ((a), (b))
#define __builtin_unpack_vector_0(a) __builtin_unpack_vector_int128 ((vector __int128_t)(a), 0)
#define __builtin_unpack_vector_1(a) __builtin_unpack_vector_int128 ((vector __int128_t)(a), 1)
#endif

/* When we have a load-store in a single-dispatch group and address overlap
 * such that forward is not allowed (load-hit-store) the group must be flushed.
 * A group ending NOP prevents the flush.
 */
#define GROUP_ENDING_NOP asm("ori 2,2,0" ::: "memory")

#if defined(__BIG_ENDIAN__) && defined (REFLECT)
#define BYTESWAP_DATA
#elif defined(__LITTLE_ENDIAN__) && !defined(REFLECT)
#define BYTESWAP_DATA
#endif

#ifdef BYTESWAP_DATA
#define VEC_PERM(vr, va, vb, vc) vr = vec_perm(va, vb,\
			(__vector unsigned char) vc)
#if defined(__LITTLE_ENDIAN__)
/* Byte reverse permute constant LE. */
static const __vector unsigned long long vperm_const
	__attribute__ ((aligned(16))) = { 0x08090A0B0C0D0E0FUL,
			0x0001020304050607UL };
#else
static const __vector unsigned long long vperm_const
	__attribute__ ((aligned(16))) = { 0x0F0E0D0C0B0A0908UL,
			0X0706050403020100UL };
#endif
#else
#define VEC_PERM(vr, va, vb, vc)
#endif

static unsigned int __attribute__ ((aligned (32)))
__crc32_vpmsum(unsigned int crc, const void* p, unsigned long len) {

	const __vector unsigned long long vzero = {0,0};
	const __vector unsigned long long vones = {0xffffffffffffffffUL,
		0xffffffffffffffffUL};

#ifdef REFLECT
	__vector unsigned char vsht_splat;
	const __vector unsigned long long vmask_32bit =
		(__vector unsigned long long)vec_sld((__vector unsigned char)vzero,
			(__vector unsigned char)vones, 4);
#endif

	const __vector unsigned long long vmask_64bit =
		(__vector unsigned long long)vec_sld((__vector unsigned char)vzero,
			(__vector unsigned char)vones, 8);

	__vector unsigned long long vcrc;

	__vector unsigned long long vconst1, vconst2;

	/* vdata0-vdata7 will contain our data (p). */
	__vector unsigned long long vdata0, vdata1, vdata2, vdata3, vdata4,
		vdata5, vdata6, vdata7;

	/* v0-v7 will contain our checksums */
	__vector unsigned long long v0 = {0,0};
	__vector unsigned long long v1 = {0,0};
	__vector unsigned long long v2 = {0,0};
	__vector unsigned long long v3 = {0,0};
	__vector unsigned long long v4 = {0,0};
	__vector unsigned long long v5 = {0,0};
	__vector unsigned long long v6 = {0,0};
	__vector unsigned long long v7 = {0,0};


	/* Vector auxiliary variables. */
	__vector unsigned long long va0, va1, va2, va3, va4, va5, va6, va7;

	unsigned int result = 0;
	unsigned int offset; /* Constant table offset. */

	unsigned long i; /* Counter. */
	unsigned long chunks;

	unsigned long block_size;
	int next_block = 0;

	/* Align by 128 bits. The last 128 bit block will be processed at end. */
	unsigned long length = len & 0xFFFFFFFFFFFFFF80UL;

#ifdef REFLECT
	vcrc = (__vector unsigned long long)__builtin_pack_vector(0UL, crc);
#else
	vcrc = (__vector unsigned long long)__builtin_pack_vector(crc, 0UL);

	/* Shift into top 32 bits */
	vcrc = (__vector unsigned long long)vec_sld((__vector unsigned char)vcrc,
        (__vector unsigned char)vzero, 4);
#endif

	/* Short version. */
	if (len < 256) {
		/* Calculate where in the constant table we need to start. */
		offset = 256 - len;

		vconst1 = vec_ld(offset, vcrc_short_const);
		vdata0 = vec_ld(0, (__vector unsigned long long*) p);
		VEC_PERM(vdata0, vdata0, vconst1, vperm_const);

		/* xor initial value*/
		vdata0 = vec_xor(vdata0, vcrc);

		vdata0 = (__vector unsigned long long) __builtin_crypto_vpmsumw
				((__vector unsigned int)vdata0, (__vector unsigned int)vconst1);
		v0 = vec_xor(v0, vdata0);

		for (i = 16; i < len; i += 16) {
			vconst1 = vec_ld(offset + i, vcrc_short_const);
			vdata0 = vec_ld(i, (__vector unsigned long long*) p);
			VEC_PERM(vdata0, vdata0, vconst1, vperm_const);
			vdata0 = (__vector unsigned long long) __builtin_crypto_vpmsumw
				((__vector unsigned int)vdata0, (__vector unsigned int)vconst1);
			v0 = vec_xor(v0, vdata0);
		}
	} else {

		/* Load initial values. */
		vdata0 = vec_ld(0, (__vector unsigned long long*) p);
		vdata1 = vec_ld(16, (__vector unsigned long long*) p);

		VEC_PERM(vdata0, vdata0, vdata0, vperm_const);
		VEC_PERM(vdata1, vdata1, vdata1, vperm_const);

		vdata2 = vec_ld(32, (__vector unsigned long long*) p);
		vdata3 = vec_ld(48, (__vector unsigned long long*) p);

		VEC_PERM(vdata2, vdata2, vdata2, vperm_const);
		VEC_PERM(vdata3, vdata3, vdata3, vperm_const);

		vdata4 = vec_ld(64, (__vector unsigned long long*) p);
		vdata5 = vec_ld(80, (__vector unsigned long long*) p);

		VEC_PERM(vdata4, vdata4, vdata4, vperm_const);
		VEC_PERM(vdata5, vdata5, vdata5, vperm_const);

		vdata6 = vec_ld(96, (__vector unsigned long long*) p);
		vdata7 = vec_ld(112, (__vector unsigned long long*) p);

		VEC_PERM(vdata6, vdata6, vdata6, vperm_const);
		VEC_PERM(vdata7, vdata7, vdata7, vperm_const);

		/* xor in initial value */
		vdata0 = vec_xor(vdata0, vcrc);

		p = (char *)p + 128;

		do {
			/* Checksum in blocks of MAX_SIZE. */
			block_size = length;
			if (block_size > MAX_SIZE) {
				block_size = MAX_SIZE;
			}

			length = length - block_size;

			/*
			* Work out the offset into the constants table to start at. Each
			* constant is 16 bytes, and it is used against 128 bytes of input
			* data - 128 / 16 = 8
			*/
			offset = (MAX_SIZE/8) - (block_size/8);
			/* We reduce our final 128 bytes in a separate step */
			chunks = (block_size/128)-1;

		    vconst1 = vec_ld(offset, vcrc_const);

			va0 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata0,
						(__vector unsigned long long)vconst1);
			va1 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata1,
						(__vector unsigned long long)vconst1);
			va2 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata2,
						(__vector unsigned long long)vconst1);
			va3 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata3,
						(__vector unsigned long long)vconst1);
			va4 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata4,
						(__vector unsigned long long)vconst1);
			va5 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata5,
						(__vector unsigned long long)vconst1);
			va6 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata6,
						(__vector unsigned long long)vconst1);
			va7 = __builtin_crypto_vpmsumd ((__vector unsigned long long)vdata7,
						(__vector unsigned long long)vconst1);

			if (chunks > 1) {
				offset += 16;
				vconst2 = vec_ld(offset, vcrc_const);
				GROUP_ENDING_NOP;

				vdata0 = vec_ld(0, (__vector unsigned long long*) p);
				VEC_PERM(vdata0, vdata0, vdata0, vperm_const);

				vdata1 = vec_ld(16, (__vector unsigned long long*) p);
				VEC_PERM(vdata1, vdata1, vdata1, vperm_const);

				vdata2 = vec_ld(32, (__vector unsigned long long*) p);
				VEC_PERM(vdata2, vdata2, vdata2, vperm_const);

				vdata3 = vec_ld(48, (__vector unsigned long long*) p);
				VEC_PERM(vdata3, vdata3, vdata3, vperm_const);

				vdata4 = vec_ld(64, (__vector unsigned long long*) p);
				VEC_PERM(vdata4, vdata4, vdata4, vperm_const);

				vdata5 = vec_ld(80, (__vector unsigned long long*) p);
				VEC_PERM(vdata5, vdata5, vdata5, vperm_const);

				vdata6 = vec_ld(96, (__vector unsigned long long*) p);
				VEC_PERM(vdata6, vdata6, vdata6, vperm_const);

				vdata7 = vec_ld(112, (__vector unsigned long long*) p);
				VEC_PERM(vdata7, vdata7, vdata7, vperm_const);

				p = (char *)p + 128;

				/*
				 * main loop. We modulo schedule it such that it takes three
				 * iterations to complete - first iteration load, second
				 * iteration vpmsum, third iteration xor.
				 */
				for (i = 0; i < chunks-2; i++) {
					vconst1 = vec_ld(offset, vcrc_const);
					offset += 16;
					GROUP_ENDING_NOP;

					v0 = vec_xor(v0, va0);
					va0 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata0, (__vector unsigned long long)vconst2);
					vdata0 = vec_ld(0, (__vector unsigned long long*) p);
					VEC_PERM(vdata0, vdata0, vdata0, vperm_const);
					GROUP_ENDING_NOP;

					v1 = vec_xor(v1, va1);
					va1 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata1, (__vector unsigned long long)vconst2);
					vdata1 = vec_ld(16, (__vector unsigned long long*) p);
					VEC_PERM(vdata1, vdata1, vdata1, vperm_const);
					GROUP_ENDING_NOP;

					v2 = vec_xor(v2, va2);
					va2 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata2, (__vector unsigned long long)vconst2);
					vdata2 = vec_ld(32, (__vector unsigned long long*) p);
					VEC_PERM(vdata2, vdata2, vdata2, vperm_const);
					GROUP_ENDING_NOP;

					v3 = vec_xor(v3, va3);
					va3 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata3, (__vector unsigned long long)vconst2);
					vdata3 = vec_ld(48, (__vector unsigned long long*) p);
					VEC_PERM(vdata3, vdata3, vdata3, vperm_const);

					vconst2 = vec_ld(offset, vcrc_const);
					GROUP_ENDING_NOP;

					v4 = vec_xor(v4, va4);
					va4 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata4, (__vector unsigned long long)vconst1);
					vdata4 = vec_ld(64, (__vector unsigned long long*) p);
					VEC_PERM(vdata4, vdata4, vdata4, vperm_const);
					GROUP_ENDING_NOP;

					v5 = vec_xor(v5, va5);
					va5 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata5, (__vector unsigned long long)vconst1);
					vdata5 = vec_ld(80, (__vector unsigned long long*) p);
					VEC_PERM(vdata5, vdata5, vdata5, vperm_const);
					GROUP_ENDING_NOP;

					v6 = vec_xor(v6, va6);
					va6 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata6, (__vector unsigned long long)vconst1);
					vdata6 = vec_ld(96, (__vector unsigned long long*) p);
					VEC_PERM(vdata6, vdata6, vdata6, vperm_const);
					GROUP_ENDING_NOP;

					v7 = vec_xor(v7, va7);
					va7 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata7, (__vector unsigned long long)vconst1);
					vdata7 = vec_ld(112, (__vector unsigned long long*) p);
					VEC_PERM(vdata7, vdata7, vdata7, vperm_const);

					p = (char *)p + 128;
				}

				/* First cool down*/
				vconst1 = vec_ld(offset, vcrc_const);
				offset += 16;

				v0 = vec_xor(v0, va0);
				va0 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata0, (__vector unsigned long long)vconst1);
				GROUP_ENDING_NOP;

				v1 = vec_xor(v1, va1);
				va1 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata1, (__vector unsigned long long)vconst1);
				GROUP_ENDING_NOP;

				v2 = vec_xor(v2, va2);
				va2 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata2, (__vector unsigned long long)vconst1);
				GROUP_ENDING_NOP;

				v3 = vec_xor(v3, va3);
				va3 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata3, (__vector unsigned long long)vconst1);
				GROUP_ENDING_NOP;

				v4 = vec_xor(v4, va4);
				va4 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata4, (__vector unsigned long long)vconst1);
				GROUP_ENDING_NOP;

				v5 = vec_xor(v5, va5);
				va5 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata5, (__vector unsigned long long)vconst1);
				GROUP_ENDING_NOP;

				v6 = vec_xor(v6, va6);
				va6 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata6, (__vector unsigned long long)vconst1);
				GROUP_ENDING_NOP;

				v7 = vec_xor(v7, va7);
				va7 = __builtin_crypto_vpmsumd ((__vector unsigned long
							long)vdata7, (__vector unsigned long long)vconst1);
			}/* else */

			/* Second cool down. */
			v0 = vec_xor(v0, va0);
			v1 = vec_xor(v1, va1);
			v2 = vec_xor(v2, va2);
			v3 = vec_xor(v3, va3);
			v4 = vec_xor(v4, va4);
			v5 = vec_xor(v5, va5);
			v6 = vec_xor(v6, va6);
			v7 = vec_xor(v7, va7);

#ifdef REFLECT
			/*
			 * vpmsumd produces a 96 bit result in the least significant bits
			 * of the register. Since we are bit reflected we have to shift it
			 * left 32 bits so it occupies the least significant bits in the
			 * bit reflected domain.
			 */
			v0 = (__vector unsigned long long)vec_sld((__vector unsigned char)v0,
					(__vector unsigned char)vzero, 4);
			v1 = (__vector unsigned long long)vec_sld((__vector unsigned char)v1,
					(__vector unsigned char)vzero, 4);
			v2 = (__vector unsigned long long)vec_sld((__vector unsigned char)v2,
					(__vector unsigned char)vzero, 4);
			v3 = (__vector unsigned long long)vec_sld((__vector unsigned char)v3,
					(__vector unsigned char)vzero, 4);
			v4 = (__vector unsigned long long)vec_sld((__vector unsigned char)v4,
					(__vector unsigned char)vzero, 4);
			v5 = (__vector unsigned long long)vec_sld((__vector unsigned char)v5,
					(__vector unsigned char)vzero, 4);
			v6 = (__vector unsigned long long)vec_sld((__vector unsigned char)v6,
					(__vector unsigned char)vzero, 4);
			v7 = (__vector unsigned long long)vec_sld((__vector unsigned char)v7,
					(__vector unsigned char)vzero, 4);
#endif

			/* xor with the last 1024 bits. */
			va0 = vec_ld(0, (__vector unsigned long long*) p);
			VEC_PERM(va0, va0, va0, vperm_const);

			va1 = vec_ld(16, (__vector unsigned long long*) p);
			VEC_PERM(va1, va1, va1, vperm_const);

			va2 = vec_ld(32, (__vector unsigned long long*) p);
			VEC_PERM(va2, va2, va2, vperm_const);

			va3 = vec_ld(48, (__vector unsigned long long*) p);
			VEC_PERM(va3, va3, va3, vperm_const);

			va4 = vec_ld(64, (__vector unsigned long long*) p);
			VEC_PERM(va4, va4, va4, vperm_const);

			va5 = vec_ld(80, (__vector unsigned long long*) p);
			VEC_PERM(va5, va5, va5, vperm_const);

			va6 = vec_ld(96, (__vector unsigned long long*) p);
			VEC_PERM(va6, va6, va6, vperm_const);

			va7 = vec_ld(112, (__vector unsigned long long*) p);
			VEC_PERM(va7, va7, va7, vperm_const);

			p = (char *)p + 128;

			vdata0 = vec_xor(v0, va0);
			vdata1 = vec_xor(v1, va1);
			vdata2 = vec_xor(v2, va2);
			vdata3 = vec_xor(v3, va3);
			vdata4 = vec_xor(v4, va4);
			vdata5 = vec_xor(v5, va5);
			vdata6 = vec_xor(v6, va6);
			vdata7 = vec_xor(v7, va7);

			/* Check if we have more blocks to process */
			next_block = 0;
			if (length != 0) {
				next_block = 1;

			    /* zero v0-v7 */
				v0 = vec_xor(v0, v0);
				v1 = vec_xor(v1, v1);
				v2 = vec_xor(v2, v2);
				v3 = vec_xor(v3, v3);
				v4 = vec_xor(v4, v4);
				v5 = vec_xor(v5, v5);
				v6 = vec_xor(v6, v6);
				v7 = vec_xor(v7, v7);
			}
			length = length + 128;

		} while (next_block);

		/* Calculate how many bytes we have left. */
		length = (len & 127);

		/* Calculate where in (short) constant table we need to start. */
		offset = 128 - length;

		v0 = vec_ld(offset, vcrc_short_const);
		v1 = vec_ld(offset + 16, vcrc_short_const);
		v2 = vec_ld(offset + 32, vcrc_short_const);
		v3 = vec_ld(offset + 48, vcrc_short_const);
		v4 = vec_ld(offset + 64, vcrc_short_const);
		v5 = vec_ld(offset + 80, vcrc_short_const);
		v6 = vec_ld(offset + 96, vcrc_short_const);
		v7 = vec_ld(offset + 112, vcrc_short_const);

		offset += 128;

		v0 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata0,(__vector unsigned int)v0);
		v1 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata1,(__vector unsigned int)v1);
		v2 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata2,(__vector unsigned int)v2);
		v3 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata3,(__vector unsigned int)v3);
		v4 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata4,(__vector unsigned int)v4);
		v5 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata5,(__vector unsigned int)v5);
		v6 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata6,(__vector unsigned int)v6);
		v7 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata7,(__vector unsigned int)v7);

		/* Now reduce the tail (0-112 bytes). */
		for (i = 0; i < length; i+=16) {
			vdata0 = vec_ld(i,(__vector unsigned long long*)p);
			VEC_PERM(vdata0, vdata0, vdata0, vperm_const);
			va0 = vec_ld(offset + i,vcrc_short_const);
			va0 = (__vector unsigned long long)__builtin_crypto_vpmsumw (
			(__vector unsigned int)vdata0,(__vector unsigned int)va0);
			v0 = vec_xor(v0, va0);
		}

		/* xor all parallel chunks together. */
		v0 = vec_xor(v0, v1);
		v2 = vec_xor(v2, v3);
		v4 = vec_xor(v4, v5);
		v6 = vec_xor(v6, v7);

		v0 = vec_xor(v0, v2);
		v4 = vec_xor(v4, v6);

		v0 = vec_xor(v0, v4);
	}

	/* Barrett Reduction */
	vconst1 = vec_ld(0, v_Barrett_const);
	vconst2 = vec_ld(16, v_Barrett_const);

	v1 = (__vector unsigned long long)vec_sld((__vector unsigned char)v0,
			(__vector unsigned char)v0, 8);
	v0 = vec_xor(v1,v0);

#ifdef REFLECT
	/* shift left one bit */
	vsht_splat = vec_splat_u8 (1);
	v0 = (__vector unsigned long long)vec_sll ((__vector unsigned char)v0,
			vsht_splat);
#endif

	v0 = vec_and(v0, vmask_64bit);

#ifndef REFLECT

	/*
	 * Now for the actual algorithm. The idea is to calculate q,
	 * the multiple of our polynomial that we need to subtract. By
	 * doing the computation 2x bits higher (ie 64 bits) and shifting the
	 * result back down 2x bits, we round down to the nearest multiple.
	 */

	/* ma */
	v1 = __builtin_crypto_vpmsumd ((__vector unsigned long long)v0,
			(__vector unsigned long long)vconst1);
	/* q = floor(ma/(2^64)) */
	v1 = (__vector unsigned long long)vec_sld ((__vector unsigned char)vzero,
			(__vector unsigned char)v1, 8);
	/* qn */
	v1 = __builtin_crypto_vpmsumd ((__vector unsigned long long)v1,
			(__vector unsigned long long)vconst2);
	/* a - qn, subtraction is xor in GF(2) */
	v0 = vec_xor (v0, v1);
	/*
	 * Get the result into r3. We need to shift it left 8 bytes:
	 * V0 [ 0 1 2 X ]
	 * V0 [ 0 X 2 3 ]
	 */
	result = __builtin_unpack_vector_1 (v0);
#else

	/*
	 * The reflected version of Barrett reduction. Instead of bit
	 * reflecting our data (which is expensive to do), we bit reflect our
	 * constants and our algorithm, which means the intermediate data in
	 * our vector registers goes from 0-63 instead of 63-0. We can reflect
	 * the algorithm because we don't carry in mod 2 arithmetic.
	 */

	/* bottom 32 bits of a */
	v1 = vec_and(v0, vmask_32bit);

	/* ma */
	v1 = __builtin_crypto_vpmsumd ((__vector unsigned long long)v1,
			(__vector unsigned long long)vconst1);

	/* bottom 32bits of ma */
	v1 = vec_and(v1, vmask_32bit);
	/* qn */
	v1 = __builtin_crypto_vpmsumd ((__vector unsigned long long)v1,
			(__vector unsigned long long)vconst2);
	/* a - qn, subtraction is xor in GF(2) */
	v0 = vec_xor (v0, v1);

	/*
	 * Since we are bit reflected, the result (ie the low 32 bits) is in
	 * the high 32 bits. We just need to shift it left 4 bytes
	 * V0 [ 0 1 X 3 ]
	 * V0 [ 0 X 2 3 ]
	 */

	/* shift result into top 64 bits of */
	v0 = (__vector unsigned long long)vec_sld((__vector unsigned char)v0,
        (__vector unsigned char)vzero, 4);

	result = __builtin_unpack_vector_0 (v0);
#endif

	return result;
}
